I am having the same problem but with a wired connection. I always gets Internet Connection: Disconnected under network settings. If i try to force a check for an update under system options, I get the following message:
"The Device Could Not Download Updates"
"System Updates could Not check for updates because of a network problem. Please check your network connections."
The router I am using is a D-Link ebr-2310 Hardware Ver: a2 Firmware: 1.05
I have added the IP of the DSM-750 to the DMZ with the same results. I use TVersity to browse my material, but have used D-Links Media Server also, and both work with no problems. Do I possibly have a defective unit? I know there was an issue with some using windows media center and the extender feature, but I don't use that, so I am not sure if it could be related to the same problem. Thanks.
PS I also have a DSM-520 that is using a wired connection. It connects to the internet with no problems and can find it's updates when available.